A. Field of Invention
The subject invention pertains to implantable cardioversion devices, or ICDs, and more particularly, to an ICD with means for applying antitachycardia therapy based on both ventricular rate and stability. The term `ICD` is used generically herein to cover implantable devices capable of providing antitachycardia therapy, including both cardioversion and defibrillation therapy. These devices may include means for providing bradycardia therapy as well.
B. Description of the Invention
Implantable cardioverison devices of the kind described above are used to provide therapy to a patient having faster than normal, often irregular heart rhythms. One such device of this type is described, for example, in U.S. Pat. No. 5,191,884, incorporated herein by reference.
Typically, in such devices, the intrinsic cardiac activity in the ventricle is monitored to detect an abnormal ventricular heart rate. It is customary to classify this heart rate into three ranges: normal, fast and very fast. Of course, for cardiac activity in the normal range, no therapy is required. For fast ventricular rate, antitachycardia (ATP) pacing may be applied. This may consist, for example, of applying pacing pulses to the heart at a rate slightly faster than the sensed VT rate (orthorhythmic pacing).
A problem with present devices is that they rely on the intrinsic ventricular rate to determine what kind of therapy to apply. However, a heart exhibiting what appears to be a ventricular rate near the border between the fast and very fast rate may not respond necessarily favorably to ATP and may require defibrillation therapy (ST). In fact, in some cases, if ST is required, ATP may cause the ventricular rate to rise even higher, and therefore it would take a longer time to stabilize the heart. There are a number of situations in which it is undesirable to apply ATP. One of these is the situation where sensing errors produce an unreliable rate estimate, causing inappropriate orthorhythmic pacing rate. Others are polymorphic ventricular tachycardia, and ventricular fibrillation.
A solution to some of these problems is attempted in U.S. Pat. No. 5,462,060. This patent discloses a dual chamber system in which interval and stability criteria are used in both cardiac chambers, as well as the PR interval to distinguish VT from arrhythmia of atrial origin, such as sinus tachycardia, atrial tachycardia, atrial flutter, atrial fibrillation, etc. These criteria are used in a complex algorithm to classify arrhythmia, while the present invention is directed toward choosing the correct therapy itself.